Multi-antenna transmission also referred to as multiple-input multiple-output (MIMO) antenna transmission allows for significant improvements in spectrum efficiency achieved over a wireless link. Using an antenna array comprising more than one antenna at a transmitter further increases a spatial degree of freedom. An antenna array may be controlled by a controller to form an antenna beam towards a receiver. An antenna array may further be controlled by the controller to achieve multiplexing gain by supporting multiple receivers concurrently. In other words, a MIMO communication system may use a plurality of channels in a spatial area concurrently. The capacity of data transmission may be increased through MIMO transmission schemes.
Two types of MIMO transmission schemes are known. A single-user MIMO (SU-MIMO) is used to transmit one data stream to a single user or a single receiver over a time-frequency resource. A multi-user MIMO (MU-MIMO) may transmit one data stream per user to at least two co-scheduled users or receivers over a single, i.e. a same time-frequency resource. MIMO technique has been adopted in 3GPP long term evolution (LTE) systems. In LTE advanced (LTE-A) standard up to eight transmit antennas forming an antenna array are admitted.
So-called massive MIMO systems or large-scale MIMO systems may employ a large number of transmit antennas, e.g. tens or even more than 100 antennas in an antenna array. Massive MIMO is under discussion for 5GPP, the 5th generation project. Massive MIMO or large scale antenna systems are capable of obtaining a further improvement of frequency efficiency.
There is a need to enhance multi-user MIMO communication especially employing massive MIMO transmitters.